What action set off the crisis in the balkans?

The crisis in the Balkans was set off by the assassination of Archduke Franz Ferdinand of Austria-Hungary on June 28, 1914, in Sarajevo. This event heightened existing tensions among the Balkan states, fueled nationalist sentiments, and triggered a series of alliances and mobilizations. The assassination led Austria-Hungary to issue an ultimatum to Serbia, ultimately resulting in the outbreak of World War I, which significantly reshaped the region. The crisis was rooted in ethnic conflicts, territorial disputes, and the decline of empires in the area.